For many applications, it is of basic importance to measure the angle of rotation of the rotating object. In general, the rotating angle of the rotating object is measured in relation to a stationary object, to which a measuring unit is attached. For example, the rotating object may be one that turns with the rotating wheel of a motor relative to a stationary machine part. The measuring unit may be both incremental and absolute. Here it is desirable to perform a measurement that is as insensitive to tolerance as possible, but one that also has a high degree of accuracy. In the ideal case, the measurement is performed without contact, in order to avoid mechanical wear.
EP 2 187 178 A1 discloses a principle of measurement that uses the optical polarization of light. To measure the rotating angle of two objects rotating opposite each other, a transmitter emits at least two light rays that are polarized in linear fashion and whose polarization planes are rotated in relation to each other. The luminosity of the light rays is modulated in phase-shifted fashion, each relative to the other. The light passes through a polarization filter, which rotates opposite the transmitter, as dependent on the rotating angle. The luminosity of the light passing through the polarization filter is measured by a receiver and is evaluated or plotted as a signal dependent on the rotating angle. The disclosed device is therefore based on both the electrical and the optical modulation of a transmission signal coming from a number of electric light sources. A disadvantage of this device derives from the use of a plurality of light transmitters, both because the reception signal can only be reduced to a linear relationship to the rotating angle when all light sources shine with precisely the same mathematical luminosity, and because productions costs are increased by the use of several light sources. If the distance between the objects rotating relative to each other undergoes changes during a rotation due to the axial tolerances of the shaft, the change in angle results in amplitude fluctuations in the signal, thereby leading to angular errors. This is conditioned by the fact that polarization is also dependent on the light ray's angle of incidence upon the surface of a polarizer—which means that a large and desirable axial tolerance in the range greater than one millimeter can be partially lost.
The goal of the invention, therefore, is to specify an improved device for measuring the rotating angle of two objects rotating relative to each other, as well to specify an improved process for measuring the rotating angle of two objects rotating relative to each other, such that both device and process provide a savings in cost.
The goal of the invention is achieved by a device with the features of patent claim 1 and by a process with the features of patent claim 13.
Advantageous embodiments and elaborations of the invention are indicated in the dependent claims.
The device according to the invention for measuring the rotating angle of two objects rotating in relation to each other—with a transmitter which is assigned to one of the objects and which emits light that is either polarized or becomes polarized by means of a polarization filter, and with a polarization-sensitive polarizer, such that the transmitter and the polarizer rotate relative to each other as dependent on the rotating angle, and with a receiver which measures the luminosity of light passing through the polarizer in order to create a signal that is dependent on the rotating angle—is distinguished by the fact that the receiver has at least two reception elements which detect light of differing polarization. The device according to the invention permits the use of both an unpolarized light source, e.g., an LED, whose light is then polarized by a polarizing filter, and a polarized light source, e.g., a laser. Since the differing polarizations are only detected in the receiver, the rotating direction is detected between the rotating object and the receiver—a fact which permits the use of a single light source and which thereby confers on the device a savings in cost. The use of at least two reception elements, and ideally a multitude of reception elements, improves the accuracy in measuring the rotating angle, inasmuch as averaging is performed over different optical channels.
In accordance with a preferred embodiment of the invention, a polarizing filter is positioned in front of each of the reception elements, and the polarization planes of the polarizing filters are rotated relative to each other. Here the number of reception elements specifically determines the number of optical channels. This arrangement can be realized in a cost-effective manner.
In principal, the polarization planes of the reception elements can be rotated at any desired angle, one relative to the other, and in particular, the angles can be taught-in. According to an advantageous embodiment of the invention, the polarization planes of the reception elements are each rotated 180°/n relative to the other, where n is the number of reception elements. This provides a uniform distribution over all directions of polarization.
The receiver advantageously exhibits at least one group, and ideally a plurality of groups, with four reception elements each, where the polarization planes of two each of the four reception elements are rotated relative to each other, specifically by 45°. When a number of channels is used, the resolution can be additionally improved by statistical averaging. In particular, it is possible also to teach-in reproducible errors.
When there are large distances between the rotating shaft and the receiver, the aperture can be enlarged by optical imaging systems, and without having to enlarge the costly area of the detectors.
Advantageously positioned in front of the receiver are at least two wedge-shaped optical elements, and ideally a number of wedge-shaped optical reception elements that corresponds to the number of reception elements. The surfaces of these wedge-shaped optical reception elements are positioned at a right angle to each other, and the polarization filters are positioned on these surfaces. Here the polarization planes of the polarization filters are rotated relative to each other. The optical elements guide an incident light ray in different directions. A lens or other imaging element is advantageously provided and focuses the rays in different directions. For example, it is possible to merge the rays running in different directions into focal points arranged on a line. A reception element can be positioned at each focal point, and such elements can be arranged to form a line array. This configuration allows light of differing polarizations to be provided to the different reception elements in a simple manner.
Advantageously positioned in front of the receiver are of strips of polarization filters, specifically N number of strips—whose polarization planes are rotated relative to each other. Positioned on these strips are strips of phase plates, specifically M number of strips—whose phases are each shifted, one relative to the next. These phase plate strips are positioned at a right angle to the polarization filter strips, and this configuration is advantageously placed on a plurality of wedge-shaped optical elements whose surfaces are positioned at a right angle to each other. With this arrangement there arises a plurality—specifically N×M—of different polarization states. Particularly preferred is N=M.
According to a preferred embodiment of the invention, a doubly refracting element, ideally a Wollaston prism, is positioned in front of the receiver, and beyond it there is advantageously placed an imaging lens system. The doubly refracting element produces two light rays having two polarization directions, one perpendicular to the other. The amplitudes of these light rays are dependent on the incident direction of polarization. With the doubly refracting element the total incident luminosity is split—which means that the two perpendicular directions of polarization equally experience fluctuations in the original amplitude. Thus, the configuration makes it possible to scale the signals to the original amplitude. The two perpendicular directions of polarization can be guided to different reception elements, e.g., by the imaging lens system, and this makes it possible to produce two different optical channels in a simple manner. Since the two optical channels differ only by a sign (sin, −sin), at least two prisms, whose optical axes are rotated at an angle one to the other, must be used to determine the position. Otherwise only the rotational speed can be directly obtained from the frequency of the electrical signals (tacho applications).
According to a particularly preferred embodiment of the invention, the polarizer has at least one phase plate, which is ideally designed as a λ/4 plate or a λ/2 plate—depending on whether the configuration is operated in transmission or reflection mode. Phase plates are more temperature-stable (frequently up to 200° C.) than organic polarizing films and produce a frequency doubling of the electrical signal. This, in turn, provides an improved angular resolution. Moreover, the transmitted luminosity is greater than that of a linear polarizer, which lets through only half of the luminosity.
The transmitter and the receiver are advantageously supplied with the same electrical modulation frequency in order to reduce sensitivity to electrical offsets, e.g., those caused by interfering light or dark currents.
Advantageously positioned behind the polarizer is a reflector, e.g., a mirror or a diffusely reflecting element, such that the reflector is positioned perpendicular to the rotating axis. The reflector advantageously rotates along with the polarizer. When a reflector is used in combination with a phase plate, a λ/4 plate is employed, whereas a phase plate of λ/2 is used for operation in transmission mode without a reflector.
In accordance with an advantageous embodiment of the invention, a beam splitter is positioned in the light path, specifically a beam splitter which does not have a polarizing design and which is provided both to cut out lateral light and to equalize the axial tolerances. Furthermore, a portion of the initial luminosity can be measured in order to identify drifts in the initial amplitude over time and, if necessary, to regulate them.
Advantageously provided are two transmitters, which are symmetrically positioned relative to the optical axis of the receiver and which are provided so as to increase the luminosity striking the receiver and to reduce the angular incidence of the light.
The process according to the invention for measuring the rotating angle between two objects rotating in relation to each other—where a transmitter is assigned to one of the objects and emits light that is either polarized or becomes polarized by means of a polarization filter, such that this light passes through a polarizer and strikes a receiver, and where the transmitter and the polarizer rotate relative to each other as dependent on the rotating angle, and where the luminosity measured by the receiver is plotted as a signal dependent on the rotating angle—is distinguished by the fact that the receiver has at least two reception elements which detect light of differing polarization.
The reception signals of those reception elements which have identical polarization planes are advantageously averaged in order to improve the accuracy of measurement.
According to a preferred embodiment of the invention, the reception signals of those reception elements which have different polarization planes are averaged, with due allowance made for the phase differences.